This application is related to Japanese application No. HEI 11 (1999)-73844 filed on Mar. 18, 1999, whose priority is claimed under 35 USC xc2xa7119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a method of manufacturing a surface acoustic wave (SAW) device. More particularly, the present invention relates to a method of reliably forming the electrode of the SAW device having a high-power durability by a reactive ion etching method.
2. Description of the Related Arts
If high power voltage is applied to an electrode constituting the SAW device, migration produces in the electrode to degrade a filter characteristic. As a countermeasure against this, it has been known that a laminated body including an Al alloy or an Al layer can be used so as to improve the high-power durability (for example, see Japanese Unexamined Patent Publication No. HEI 7 (1995)-122961, Japanese Unexamined Patent Publication No. HEI 10 (1998)-135767). This electrode is usually formed by dry etching such as reactive ion etching. A mixed gas comprising halogen such as BCl3 and Cl2 is generally used for the etching (for example, see Japanese Unexamined Patent Publication No. HEI 8 (1996)-264508, Japanese Unexamined Patent Publication No. HEI 9 (1997)-181553).
When an electrode containing a substance such as Ti, Pd, Si, or the like, whose halide has a high vapor pressure, in addition to Al, is subjected to the dry etching, the electrode can be etched without leaving residues. However, when an electrode containing a metal such as Cu, Mg, or the like, whose halide has a low vapor pressure, is subjected to the dry etching, there is a problem that the halide remains as a residue to induce aftercorrosion, and in particular, when the electrode contains a high concentration not less than 2 wt% of these metals, there is a problem that the etching itself does not proceed.
Further, a piezoelectric substrate made of LiNbO3, LiTaO3, Li2B4O7, or the like is used for the SAW device and there is also a problem that a compound of halogen contained in a gas used for etching and lithium derived from the piezoelectric substrate also remain as residues and further increase the generation of the aftercorrosion.
Although it is thinkable that the etching rate of a layer containing the metal whose halide has a low vapor pressure and Al is increased, while reducing residues, by physical etching such as sputtering with a rare gas, the physical etching produces a problem that a side wall is reversely tapered in the direction of thickness. Further, etching is continuously performed for a long time (over-etching) so as to remove residues yet to be etched, and this produces a problem that the physical etching is poor in selectivity and hence etches also the piezoelectric substrate to degrade the characteristics of the SAW device.
A method of etching a layer containing Al with a mixed gas comprising a gas containing halogen and He is disclosed by Japanese Unexamined Patent Publication No. SHO 61 (1986)-147532, although it does not refer to the SAW device. He is used there at a high flow of 800 sccm or more so as to have an effect on cooling the layer and to stabilize plasma. However, there is a problem that if the He is used at such a high flow, a gas pressure can not be reduced and hence a selectivity to an electrode and to a resist used as a mask for the etching can not be increased.
A method of etching a layer containing Al with a mixed gas comprising Cl2 and He by ECR etching, instead of a reactive ion etching, is disclosed in Japanese Unexamined Patent Publication No. HEI 6 (1994)-333883.
The ECR etching has a problem that, when compared with the reactive ion etching, its apparatus is expensive and needs an ultra-high vacuum exhaust and hence reduces throughput. Further, the ECR etching has a problem that, because low pressure high density plasma is used in the ECR etching, in principle, an electron density is higher in the ECR etching than in the reactive ion etching by two to three orders, and hence the SAW device using the piezoelectric substrate is charged and produces defects derived therefrom.
It has been considered that since the reactive ion etching has higher incident ion energy than the ECR etching, it is poor in selectivity and hence tends to cause damage to a piezoelectric substrate which is an underlayer. However, the present inventors have unexpectedly discovered that an electrode containing the metal whose halide has a low vapor pressure and Al can be formed with a practical selectivity by etching under a gas pressure of 0.1 Pa to 3 Pa with an etching gas containing a mixed gas comprising Cl2 and He, and have made the present invention.
The present invention provides a method of manufacturing a SAW device having an electrode containing Al and the other metal, comprising; patterning a layer of an Al and other metal forming the electrode by reactive ion etching which is conducted by using an etching gas containing a mixed gas comprising Cl2 and He under a gas pressure of 0.1 Pa to 3 Pa.
These and other objects of the present application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.